SDL_SetAlpha(3) 						 SDL API Reference						   SDL_SetAlpha(3)

NAME

       SDL_SetAlpha - Adjust the alpha properties of a surface

SYNOPSIS

       #include "SDL.h"

       int SDL_SetAlpha(SDL_Surface *surface, Uint32 flag, Uint8 alpha);

DESCRIPTION

	      Note:

	      This  function and the semantics of SDL alpha blending have changed since version 1.1.4. Up until version 1.1.5, an alpha value of 0
	      was considered opaque and a value of 255 was considered transparent. This has now been inverted: 0  (SDL_ALPHA_TRANSPARENT)  is  now
	      considered transparent and 255 (SDL_ALPHA_OPAQUE) is now considered opaque.

       SDL_SetAlpha is used for setting the per-surface alpha value and/or enabling and disabling alpha blending.

       Thesurface  parameter  specifies  which	surface whose alpha attributes you wish to adjust. flags is used to specify whether alpha blending
       should be used (SDL_SRCALPHA) and whether the surface should use RLE acceleration for blitting (SDL_RLEACCEL). flags can be an OR'd  combi-
       nation  of these two options, one of these options or 0. If SDL_SRCALPHA is not passed as a flag then all alpha information is ignored when
       blitting the surface. The alpha parameter is the per-surface alpha value; a surface need not have an alpha channel to use per-surface alpha
       and blitting can still be accelerated with SDL_RLEACCEL.

	      Note:

	      The per-surface alpha value of 128 is considered a special case and is optimised, so it's much faster than other per-surface values.

       Alpha effects surface blitting in the following ways:

       RGBA->RGB with SDL_SRCALPHA
			   The	source	is  alpha-blended with the destination, using the alpha channel. SDL_SRCCOLORKEY and the per-surface alpha
			   are ignored.

       RGBA->RGB without SDL_SRCALPHA
			   The RGB data is copied from the source. The source alpha channel and the per-surface alpha value are ignored.

       RGB->RGBA with SDL_SRCALPHA
			   The source is alpha-blended with the destination using the per-surface alpha value. If SDL_SRCCOLORKEY is set, only the
			   pixels not matching the colorkey value are copied. The alpha channel of the copied pixels is set to opaque.

       RGB->RGBA without SDL_SRCALPHA
			   The RGB data is copied from the source and the alpha value of the copied pixels is set to opaque. If SDL_SRCCOLORKEY is
			   set, only the pixels not matching the colorkey value are copied.

       RGBA->RGBA with SDL_SRCALPHA
			   The source is alpha-blended with the destination using the source alpha channel. The alpha channel in  the  destination
			   surface is left untouched. SDL_SRCCOLORKEY is ignored.

       RGBA->RGBA without SDL_SRCALPHA
			   The	RGBA  data  is copied to the destination surface. If SDL_SRCCOLORKEY is set, only the pixels not matching the col-
			   orkey value are copied.

       RGB->RGB with SDL_SRCALPHA
			   The source is alpha-blended with the destination using the per-surface alpha value. If SDL_SRCCOLORKEY is set, only the
			   pixels not matching the colorkey value are copied.

       RGB->RGB without SDL_SRCALPHA
			   The	RGB data is copied from the source. If SDL_SRCCOLORKEY is set, only the pixels not matching the colorkey value are
			   copied.

	      Note:

	       Note that RGBA->RGBA blits (with SDL_SRCALPHA set) keep the alpha of the destination surface. This means that  you  cannot  compose
	      two  arbitrary RGBA surfaces this way and get the result you would expect from "overlaying" them; the destination alpha will work as
	      a mask.

	      Also note that per-pixel and per-surface alpha cannot be combined; the per-pixel alpha is always used if available

RETURN VALUE

       This function returns 0, or -1 if there was an error.

SEE ALSO

       SDL_MapRGBA, SDL_GetRGBA, SDL_DisplayFormatAlpha, SDL_BlitSurface

SDL
							      Tue 11 Sep 2001, 23:01						   SDL_SetAlpha(3)